backends/chert/chert_check.cc: Fix DBCHECK_FIX to be able to find a
level 0 root block (happens when a table has sufficiently little
data in that it all fits in one block).

git-svn-id: file:///home/xapian-svn/xapian/trunk@18327 6b2f1b92-a4f3-0310-943f-de3cbe41748b